Makch 4, 1921] 



SCIENCE 



201 



in the geologic section and rock fracture be- 

 low. On tlie whole, the oldest rocks undoubt- 

 edly show greater evidences of rock flowage, 

 though even here such evidences are localized 

 in relatively narrow and numerous zones. 

 These rocks have suffered more periods of de- 

 formation, some near the surface and some 

 deep below, than the younger rocks. The pres- 

 ent evidences of flow do not necessarily indi- 

 cate that all the flowage occurred at great 

 depths. Plutonic intrusions of great mass 

 often, not always, cause rock flowage in the 

 adjacent beds, and so far as such intrusions 

 are more numerous with depth, rock flowage 

 may increase. On the other hand, some plu- 

 tonic intrusions in younger series which have 

 not been very deeply buried likewise cause rock 

 flowage. Certain it is that shearing move- 

 ments, resulting in displacements which we 

 call faults, have extended down to the bottom 

 of our zone of observation. These partake of 

 the nature of rock fracture in their confine- 

 ment to planes and in their relations to 

 stresses, but whether the processes be called 

 flow or fraetnre is partly a matter of definition 

 to which we shall presently make further al- 

 lusion. 



II. THE UNSEEN ZONE BELOW 



Below the zone where the evidences of struc- 

 tural failure can be observed, conceptions of 

 (the structural behavior of rocks are based on 

 .such a variety of assumptions that the layman, 

 ,and for that matter tlie geologist, has much 

 difficulty in understanding and reconciling the 

 various views. It is certain that rocks fail in 

 .this zone; there is evidence which permits of 

 no other conclusion; but the manner, distri- 

 bution, and causes of this failure are by no 

 means clear. There are certain fundamental 

 facts upon which any hypothesis must be 

 built. 



Known Fads. — Tidal experiments have 

 shown that the earth as a whole is stronger 

 than steel and acts almost as an ideally rigid 

 substance. 



The behavior of earthquake waves indicates 

 that the earth behaves as a solid throughout; 

 .and for the outer quarter of the earth, at least, 



the waves increase in velocity of transmission 

 with depth, showing that elasticity and rigidity 

 increase faster than density. 

 ( Under -surface conditions a dome of the 

 strongest rock, corresponding to the sphericity 

 of the earth, has a calculated supporting 

 strength equal only to a very small fraction of 

 the dome's ovm weight; but experimental work 

 on deformation of rocks has shown that, with 

 increase of containing pressures or cubical 

 compression, the rock takes on a rigidity ca- 

 pable of resisting enormous stress differences. 

 The range of experimental evidence is not yet 

 sufficient to show the magnitude of these dif- 

 ferential stresses necessary to produce defor- 

 mation under the conditions of pressure which 

 might be reasonably inferred below our zone of 

 observation; but quoting from Adams"' "the 

 experiments seem to indicate that with a con- 

 .taining pressure of about 10,000 atmospheres, 

 which would be equivalent to a depth of about 

 ,twenty-two miles below the surface, it would 

 be impossible to make the marble flow, except 

 tinder a pressure which would be simply colos- 

 iSal." Geologic evidence seems to indicate a 

 supporting strength in the deep zone far 

 greater than that of surface rocks. 

 I The rocks in the deep zone are under higher 

 temperature and greater pressure than in the 

 ^one of observation. Some notion of the 

 quantitative values of these factots is afforded 

 .by downward extrapolation of observed gradi- 

 ents nearer the surface. 



The density of rocks within the zone of ob- 

 servation averages about 2.7; the density of 

 the earth as a whole as determined astronom- 

 ically is in round number 5. It follows, there- 

 fore, that the density of part of the earth must 

 be higher than 5, and that the density of the 

 .deep zone must be higher than at the surface ; 

 but beyond this the distribution of density in 

 the deep zone, both vertically and horizontally, 

 are unknown. 

 ^ By means of the plumb line and pendulum, 



* Adams, Frank D. and Bancroft, J". Austen, 

 "On the Amount of Internal Friction Developed 

 in Eocka during Deformation and on the Rela- 

 tive Plasticity of Different Types of Rocks, ' ' Jour. 

 Geol, Vol. 25, 1917, p. 635. 



